Doors belong to those devices that are used by us on an everyday basis. To facilitate the automatic closing of opened doors, door closers have been provided which are affixed to the doors and door frames. There are a wide variety of door closers, most notably the sliding door closers provided in departmental stores and offices which open and close automatically. However, the sliding door closers are very expensive and thus are not suitable for ordinary homes, small offices, machine shops or manufacturing plants. Spring-type door closers are much less expensive; however, they close too quickly and often cause violent collisions between doors and door frames, thus resulting in damages to the doors. There are also other spring-type door closers available in the market which are connected with a pneumatic or hydraulic shock absorber to provide a smoother door closing action. However, since the resistance provided by the pneumatic or hydraulic shock absorber is constant and the spring force is proportional to the extent to which the door is opened, the door closing speed is still too violent during the first four-fifth of the closing period, resulting an unstable door closing action and undesirable inconvenience.
Some door closers are provided with adjustable door closing speeds. For example, FIG. 1 is a perspective view of a conventional door closer with such adjustable door closing speeds. The cylinder 1 is exposed to the atmosphere and a square shoulder 3 is provided which can be turned by a wrench 2 to adjust the closing speed of the door closer. One of the shortcomings associated with the conventional adjustable door closers is that the square shoulder 3 only allows a very limited room for speed adjustment. Furthermore, because the cylinder 1 is exposed to the atmosphere, it often collects dust and adversely affects the long term durability of the door closer.
FIG. 2 shows a perspective view of another conventional door closer with adjustable door closing speeds; it is similar to the door closer shown in FIG. 1, but is provided with a plastic cover 5 formed from an extrusion process to protect the cylinder I from exposure to the atmosphere. A U-shaped square shoulder cover 5a is also provided which can be easily opened to allow a wrench 6 to work on the square shoulder 3. This type of door closer minimizes the dusting problem experienced in the first type; however, many of the shortcomings still exist. For example, it is still cumbersome to work on the speed-adjusting square shoulder 3. Furthermore, the plastic material, from which the cylinder cover 5 is made, can be fire-hazardous. The cylinder cover 5 often is distorted and/or cracked due to aging or other abuse, thus losing its intended protective purpose.